1. Field of the Invention
The present invention relates to an optical disc beneficial for reproducing, or recording and reproducing high-density, large-capacity optical discs.
2. Description of the Related Art
Currently prevailing optical discs include optical discs of the CD format such as CDs, CD-ROMs, CD-Rs and CD-RWs, and the DVD format such as DVDs, DVD-ROMs, DVD-Rs, DVD-RWs, DVD-RAMs, DVD+Rs, and DVD+RWs capable of recording and reproducing data with higher density and larger capacity by the use of red lasers. Particularly in recent years, the Blu-ray disc standard (the term “Blu-ray disc” is a trademark of SONY KABUSHIKI KAISHA CORPORATION JAPAN 7-35, Kitashinagawa 6-chome Shinagawa-ku, Tokyo JAPAN), the Advanced Optical Disc (AOD) standard and the like capable of recording and reproducing data with even higher density and larger capacity by the use of blue lasers have been introduced. Optical discs and disc drives according to such standards are being commercially developed.
As an example of a mechanism for reading out data (pickup) in such an optical disc device, a mechanism conventionally adopted for an optical disc device of the Blu-ray disc standard will be schematically described as referring to FIG. 9.
As shown in FIG. 9, an optical disc device of this kind usually comprises a pickup 50 which is a mechanism for reading a disc. The pickup 50 basically comprises a semiconductor laser oscillator (LD) 150 which oscillates the laser light, a 45-degree reflection mirror 56 which reflects the laser light, an objective lens 2 which converges the laser light to focus on a reflection layer (recording layer) 10 of the optical disc, and a polarizing beam splitter 51 which leads the reflection light from the optical disc and the 45-degree reflection mirror 56 to a light receiving element (PD) 160. A lens actuator (not shown) capable of moving the objective lens 2 slightly upward and downward is provided for focusing as an auxiliary unit. The objective lens 2 must have high quality to converge a beam to the diffraction limit, and its numerical aperture (NA) is set as high as about 0.85 for example.
In this example, the semiconductor laser oscillator 150 used as a light source is typically a blue-violet laser diode which oscillates laser light with a wavelength of 405 nm. A collimator lens 53 for shaping incident laser light is provided so that collimated light is incident to the objective lens. A condenser lens 57 and a cylindrical lens 58 are provided for condensing reflection light of laser light reflected from the reflection layer 10 of the loaded optical disc.
More specifically, predetermined polarized components out of linear polarized laser light generated from the semiconductor laser oscillator 150 are transmitted toward the disc through the polarizing beam splitter 51 in order to be circularly polarized by a quarter wavelength plate 52. Laser light collimated by the collimator lens 53 is reflected from the 45-degree reflection mirror 56 and irradiated to the reflection layer 10 after being converged by the objective lens 2. The laser light reflected from the reflection layer 10 reaches the quarter wavelength plate 52 through the 45-degree reflection mirror 56 and the collimator lens 53, to become linearly polarized light which has a phase difference of 90 degrees from the original polarizing direction. The polarizing beam splitter 51 only reflects polarized components different from the polarized components reflected as described above so that reflected light is condensed by the condenser lens 57 and the cylindrical lens 58 to be incident to the light receiving element 160. The light receiving element 160 converts incident laser light into an electrical signal. The converted electrical signal is amplified and transmitted outside the pickup 50 to be demodulated in a well-known manner.
When focusing by a pickup, the distance between the disc surface and the objective lens is adjusted by driving the lens actuator to move the objective lens upward and downward.
When recording and reproducing data on such a high-density optical disc, a focal spot diameter of laser light oscillated from the semiconductor laser oscillator (laser light source) must be small on the disc. The spot diameter is basically calculated from the following formula.Spot diameter=wavelength of laser light source λ/numerical aperture of objective lensNA   (1)
As can be seen from this formula (1), the focal spot diameter of laser light is proportional to the wavelength of the laser light source λ, and is inversely proportional to the numerical aperture of the objective lens NA. Therefore, the focal spot diameter of laser light may be reduced by shortening the wavelength of the laser light source λ or by using the objective lens with higher numerical aperture. For example, the wavelength λ of the laser light source is 405 nm while the numerical aperture of the objective lens is 0.85 for the optical disc of the Blu-ray disc standard.
If the numerical aperture of the objective lens is this high, however, tolerance for disc tilt becomes stringent. Tolerance for disc tilt is calculated from the following formula.Tolerance for disc tilt=wavelength of laser light source λ/(numerical aperture ofobjective lens NA)3   (2)
As can be seen from this formula (2), tolerance for disc tilt is proportional to the wavelength of the laser light source, and is reduced in inverse proportion to the 3rd power of the numerical aperture of the objective lens. Therefore, the thickness of the disc cover layer must be particularly small in order to maintain tolerance for disc tilt for the optical disc of the Blu-ray disc standard which utilizes the objective lens with a high numerical aperture.
The optical disc of the Blu-ray disc standard has a tolerance for disc tilt which is one-fifth of that of the DVD standard (wavelength of laser light source λ: 650 nm, numerical aperture of objective lens NA: 0.6) Therefore, the optical disc of the Blu-ray disc standard must have a cover layer of approximately 100 μm in thickness as compared to a cover layer of 600 μm in thickness of the optical disc of the DVD standard.
Additionally, two reflection layers (recording layers) are supposed to be provided on one side of the high-density optical disc in order to increase data recording capacity. The first reflection layer (recording layer) and the second reflection layer (recording layer) must be distanced from each other as far as possible (for example, about 25 μm apart) so that reflection light from one layer does not affect reflection light from the other layer. Consequently, the thickness of the cover layer from the disc surface to the first reflection layer and the thickness of the cover layer from the disc surface to the second reflection layer are different. In the optical disc of the Blu-ray disc standard where the thickness of the cover layer is particularly thin, the ratio of each thickness deviation of the cover layer for the first reflection layer and the second reflection layer to the thickness 100 μm of the cover layer of the disc increases since the thickness of the cover layer of the optical disc is inherently thin.
On the other hand, although the objective lens is designed in consideration of the thickness of the cover layer of the disc, spherical aberration is generated on reflection layers of the optical disc if the thickness of the cover layer of the disc is out of the standard thickness of 100 μm.
Next, the relation between spherical aberration and the thickness of the cover layer of the disc will be described as referring to FIG. 10. FIGS. 10A, 10B and 10C enlarge and illustrate the relation between the cover layer of the disc and the focal position respectively when the cover layer of the disc is thinner than the standard (FIG. 10A), according to the standard (FIG. 10B) and thicker than the standard (FIG. 10C).
As shown in FIG. 10A, the focal position is recognized at a position a little more distant from the disc in a focus search when the thickness of the disc cover is thinner than the standard as compared to the case when the thickness of the cover layer is according to the standard. Therefore, a focus error 9FE) signal of laser light reflected from the optical disc is recognized short of the reflection layer. The focal spot diameter of laser light becomes large on the surface of the reflection layer of the disc since rays of laser light intersect before the reflection layer of the disc, which causes spherical aberration. As shown in FIG. 10B, a focus error signal is recognized on the reflection layer of the disc in the focus search when the thickness of the disc cover is according to the standard, so that the focal position may fall on the reflection layer of the disc. On the other hand, when the thickness of the disc cover is thicker than the standard, the focal position of the objective lens is recognized at a position a little nearer to the disc in the focus search as compared to the case when the thickness of the cover layer is according to the standard, as shown in FIG. 10C. Therefore, laser light reflected from the optical disc is focused at a deeper position inside the disc away from the objective lens. The focal spot diameter of the laser light thus becomes large on the surface of the reflection layer of the disc since the focus error signal is recognized at the deeper position beyond the reflection layer, which causes spherical aberration.
Spherical aberration is basically calculated from the following formula.Spherical aberration=(thickness deviation of cover layer Δ d/standard thickness ofcover layer d)×(numerical aperture of objective lens NA)4   (3)
As can be seen from this formula (3), spherical aberration is proportional to the 4th power of the numerical aperture of the objective lens NA.
Such spherical aberration hinders the appropriate focal spot diameter from falling on the reflection layer of the optical disc, degrading recording or reproducing function of the optical disc device.
In this connection, an optical disc device to detect spherical aberration by the use of a hologram element is conventionally disclosed in the Japanese Published Application 367197/2002, A, for example. In this optical disc device, a hologram element is used to separate light into a light flux passing through the outer circumference, which is away from the optical axis, of the objective lens and a light flux passing through the center, which is close to the optical axis, of the objective lens. Spherical aberration is detected by obtaining the difference in intensity of the two light fluxes.
In this way, spherical aberration may be corrected by providing means for detecting spherical aberration in an optical disc device to control an actuator for correcting spherical aberration with a feedback of the detected spherical aberration signal.
Providing means for detecting spherical aberration as described above for an optical disc deice, however, not only complicates the detection mechanism but also increases the number of parts. Furthermore, the number of manufacturing processes and manufacturing cost inevitably increase because of the necessity of adjustment work and so on.
The present invention was made in consideration of such conditions and its objective is to provide an optical disc device which may detect factors causing spherical aberration more easily and precisely without using complicated means for detecting spherical aberration.